The molecular mechanism of integration of SV40 DNA into the cell DNA of transformed and infected permissive cells is being examined using restriction enzymes and electron microscopy. The objective is to characterize the number, organization and orientation of the viral DNA sequences within the host chromosome. BIBLIOGRAPHIC REFERENCES: Electron microscopic localization of a protein bound near the origin of SV40 DNA replication. Jack Griffith, Marianne Dieckmann and Paul Berg. J. Virol. 15, 167(1975). A biochemical procedure for the production of small deletions in Simian Virus 40 DNA. John Carbon, Thomas E. Shenk and Paul Berg. Proc. Nat. Acad. Sci. 72, 1392 (1975).